New-Mailbox 
  -DisplayName "AD-DisplayName" 
  -Name "AD-Name" 
  -FirstName "Vorname" 
  -LastName "Nachname" 
  -OrganizationalUnit "distinguishedName of OU" 
  -UserPrincipalName "AD-Username" 
  -SamAccountName "AD-SamAccountName" 
  -LinkedCredential $credential 
  -LinkedMasterAccount "dom\samaccountname" 
  -LinkedDomainController "dc" 
  -alias "mailboxalias" 
  -Database Databasename